1. Field of the Invention
The present invention relates generally to an apparatus for controlling an automatic transmission of a motor vehicle, and more particularly to improvements in such a control apparatus adapted to shift the transmission from one of operating positions to another according to a predetermined shift pattern.
2. Discussion of the Related Art
A power transmitting system of a motor vehicle generally includes an automatic transmission which has a fluid coupling device such as a torque converter which receives power from an engine of the vehicle, and an automatic speed changing device such as a planetary mechanism connected to the fluid coupling device. The automatic transmission is controlled by a suitable control apparatus, such that the transmission is shifted from one of a plurality of operating positions thereof to another, according to predetermined shift patterns.
An example of a shift pattern for an automatic transmission having four forward drive positions is illustrated in FIG. 4. This shift pattern uses two control parameters, which consist of an accelerator position .theta.ac and a running speed V of the vehicle. Solid lines in FIG. 4 indicate boundaries for shift-up operations of the transmission which cause a decrease in the speed ratio of the transmission (input speed/output speed), while broken lines in the figure indicate boundaries for shift-down operations of the transmission which cause an increase in the speed ratio. Reference numerals and characters 1, 2, 3 and O/D denote the four operating positions of the transmission, i.e., first-speed position, second-speed position, third-speed position and overdrive position, respectively. The speed ratios of these first-speed, second-speed, third-speed and overdrive positions decrease in the order of description.
Where the transmission is currently placed in the third-speed position 3 with the accelerator position .theta.ac set at 40%, for example, three reference values V1, V2, V3 are determined by the shift pattern, so that the actually detected vehicle speed V is compared with these reference values V1 V2, V3, for selecting the position to which the transmission should be shifted up or down. Described more specifically, the first-speed position 1 is selected where V .ltoreq.V1, and the second-position 2 is selected where V1&lt;V.ltoreq.V2. The third-speed position 3 is selected where V2&lt;V.ltoreq.V3, and the overdrive position O/D is selected where V3&lt;V. The operating positions may be selected by comparing the detected accelerator position .theta.ac with reference values determined by the shift pattern for each vehicle speed V.
For improving the adequacy of selection of the optimum operating position of the automatic transmission, it is considered to adjust or compensate the basic shift pattern according to compensating data maps, or use a large number of shift patterns, so that the selected position of the transmission best suits the specific vehicle running condition which is defined by two or more parameters such as: a rate of change in the accelerator position .theta.ac; a rate of change in the vehicle speed V; occurrence of repetition of frequent shift-up and shift-down actions of the transmission; a gradient of the road surface; a difference between the actual engine speed and the determined target or desired engine speed; and a steering angle of the vehicle. The compensation of the basic shift pattern or the use of many shift patterns permits optimum shifting operations of the transmission for enhanced driving comfort of the vehicle, depending upon the vehicle running condition. However, this arrangement which uses many control parameters requires a considerably increased amount of control program data (including data maps), and therefore requires a large-capacity memory for storing such data, resulting in an increase in the cost of the control apparatus. Namely, the required amount of control program data increases in proportion to the number of control parameters which is raised to n-th power.
In view of the above drawback, the assignee of the present application developed a transmission control apparatus as disclosed in U.S. Pat. application, Ser. No. 352,498 filed May 16, 1989. This control apparatus does not use shift patterns or compensating data maps, but is adapted to control the transmission, according to calculated degrees of satisfaction of predetermined fuzzy set control rules associated with running parameters of the vehicle. Namely, the satisfaction degrees of the control rules are calculated for each of the operating positions of the transmission, and the operating position having the highest satisfaction degree is selected as the position which best suits the current running condition of the vehicle. In this arrangement, the required amount of control program data is proportional to the number of the control parameters (vehicle running parameters), and is therefore comparatively small, whereby the control apparatus may be simplified and available at a relatively reduced cost.
In the above arrangement, however, the required amount of control program data is larger than that in the arrangement using only the shift patterns, in the case where the number of control parameters for selecting the operating positions of the transmission is relatively small, for example, where only the accelerator position and the vehicle speed are used as the control parameters. Thus, the conventional and recently proposed arrangements are not completely satisfactory in terms of the control reliability and simplicity.